A known fixing device includes an endless belt, a nip member, a backup member, and an upstream guide. The nip member is disposed inside a loop of the endless belt. The backup member pinches the endless belt in conjunction with the nip member to form a nip portion between the endless belt and the backup member. The upstream guide is disposed upstream of the nip portion in a rotating direction of the endless belt. The upstream guide guides rotation of the endless belt toward the nip portion. In the fixing device, more specifically, a downstream end portion of a guide surface of the upstream guide and an upstream end portion of the nip member are spaced apart from each other in a sheet conveying direction by a gap. More specifically, the gap between the downstream end portion of the guide surface of the upstream guide and the upstream end portion of the nip member in the sheet conveying direction has a constant width in a width direction of the endless belt.